1. Field of the Invention
The present invention relates to a failure diagnostic apparatus and method for a resistor element such as a squib of a passive safety system installed in an automotive air bag system or a pretensioner of an automotive seat belt system.
2. Related Art
A prior art passive safety system like an air bag system has a small actuating element such as a squib so as to be activated immediately upon an occurrence of collision by a signal upon detecting the occurrence of collision by a G sensor. A resistor is the main portion of the squib. Therefore, the squib is heated up and detonates by conducting electricity through the resistance so that the safety device is operated. Since the squib has to be heated up instantaneously, it usually has a structure like a filament of a flash lamp. Thus, it cannot have a mechanically strong structure. However, since the squib is required to work without fail at the occurrence of collision, the squib should be always checked to determine whether it is in an operable condition or in an inoperable (failure) condition.
Usually checking of the squib is made on whether a resistance value thereof is normal or not. A small monitor current for checking is conducted through the squib. A voltage at both sides of the squib is checked. The resistance value is determined from the voltage value and the resistance value is checked whether it is normal or not. Since the squib is used to produce a lot of gas by heat only upon the detection of the collision, a large amount of current cannot be conducted as the monitor current and it is limited to about 50 mA. It is impossible to flow more current since it has more risks of unnecessarily operating the squib itself. Since the resistance value of the squib has normally only about 2.OMEGA., the voltage value detected at both sides of the squib is small. Thus, an amplifier is used for amplifying the voltage value at both sides of the squib. Since the amplifier is mounted easily on a circuit substrate of the device as an operational amplifier due to a recent semiconductor manufacturing technology, the small voltage can be amplified easily.
A high performance operational amplifier is, however, high in cost, and consequently an operational amplifier usually used for a precision measuring equipment cannot be used from the standpoint of cost. Therefore, it is desired that the voltage is detected by a general purpose operational amplifier. However, since a general purpose operational amplifier produces errors such as offset voltages, such errors cannot be ignored relative to the small voltage described above. Thus, precise voltage value cannot be obtained, and the resistance value of the squib cannot be determined precisely either. Hence, it is likely that the squib, even if in the failure condition, will be determined normal erroneously, and the passive safety device may not be actuated upon the vehicle collision.